Anchor-In-Anchor System For Use In Bone Fixation

ABSTRACT

An anchor-in-anchor fixation system is provided for securing underlying structure, such as bone. The fixation system includes a first bone anchor having a shaft for fixation to underlying bone, and a head that defines an internal bore. A second bone anchor extends through the bore and into underlying bone. A fixation assembly is also provided that includes one or more fixation systems coupled to an auxiliary attachment member configured for long bone fixation, spinal fixation, or fixation of other bones as desired.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/631,293, filed Dec. 4, 2009, now pending, which claims the benefit ofU.S. Patent Application Ser. No. 61/120,138, filed Dec. 5, 2008. Theentire disclosure of each application listed in this paragraph isincorporated by reference in this application for all purposes.

TECHNICAL FIELD

The present disclosure relates generally to orthopedics, an inparticular relates to fixation systems and associated surgical methodsand procedures for using same.

BACKGROUND

A variety of fixation devices for the reduction of bone or bonefragments are well known. For instance, external bone fixation devices,or external fixators, are used to reduce fractures of the long bones inthe human body. Internal bone fixation devices, such as bone plates, arealso commonly used to reduce bone fractures. Spinal fixation devicesincluding intervertebral implants, spinal rods, and the like, are usedto replace intervertebral discs, fuse or align adjacent vertebrae, andaddress other spinal issues.

A large number of fixation devices are attached to underlying bone usingbone anchors, which can include screws, pins, nails, and the like. Forinstance, a typical bone plate includes screw holes that accommodatebone screws which are drilled into underlying bone on opposing sides ofa fracture to join bone segments together. A typical cervical spineimplant can likewise include screw holes that accommodate screws whichare drilled into adjacent vertebral bodies in order to fix the positionof the implant. Unfortunately, the attachment of fixation devices to theunderlying bone can become compromised if, for instance, the screwbecomes dislodged from the bone during normal anatomical function.

What is therefore desirable is an anchor system for use in bone fixationthat more reliably fastens a fixation device to underlying bone.

SUMMARY

An anchor-in-anchor fixation system includes a first bone anchorincluding a first shaft and a first head, the first shaft extendingalong a first longitudinal shaft axis and configured to attach tounderlying structure, and the first head defining a bore extendingtherethrough along a bore axis, wherein the bore axis and the firstlongitudinal shaft axis define an acute angle. The anchor-in-anchorfixation system further includes a second bone anchor including a secondshaft and a second head, the second shaft extending along a secondlongitudinal shaft axis and configured to attach to underlyingstructure, the second bone anchor configured to be inserted into thebore.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe preferred embodiments of the application, will be better understoodwhen read in conjunction with the appended drawings. For the purposes ofillustrating the anchor-in-anchor system, there are shown in thedrawings preferred embodiments. It should be understood, however, thatthe application is not limited to the precise arrangements andinstrumentalities illustrated in the drawings, in which:

FIG. 1 is a perspective view of an anchor-in-anchor system constructedin accordance with one embodiment and including a first bone anchorreceiving and attached to a second bone anchor;

FIG. 2A is a side elevation view of the first bone anchor of theanchor-in-anchor system illustrated in FIG. 1;

FIG. 2B is another side elevation view of the first bone anchor of theanchor-in-anchor system illustrated in FIG. 1;

FIG. 2C is an end view of the first bone anchor of the anchor-in-anchorsystem illustrated in FIG. 1 constructed in accordance with analternative embodiment;

FIG. 3 is a side elevation view of the second bone anchor of theanchor-in-anchor system illustrated in FIG. 1; FIG. 2A is;

FIG. 4A is an exploded view showing the second bone anchor aligned forinsertion in the first bone anchor;

FIG. 4B is a view similar to FIG. 4A but showing the second bone anchorpartially inserted in the first bone anchor;

FIG. 4C is a view similar to FIG. 4B but showing the second bone anchorinserted and attached to the first bone anchor;

FIG. 4D is a perspective view of the first bone anchor illustrated inFIG. 2C with a portion cut away, and further showing a second boneanchor insertable into the first bone anchor at variable angles;

FIG. 5 is a perspective view of an anchor-in-anchor fixation assemblyincluding a pair of an anchor-in-anchor fixation systems as illustratedin FIG. 1 attached to an auxiliary fixation device provided as a boneplate;

FIG. 6A is an exploded view showing a pair of first anchors fixed tounderlying bone segments;

FIG. 6B is view similar to FIG. 6A, but showing a bone plate configuredfor attachment to the first anchors;

FIG. 6C is a view similar to FIG. 6B, but showing the bone plateattached to the first anchors;

FIG. 6D is a view similar to FIG. 6C, but showing a pair of secondanchors aligned for insertion into the first anchors and the underlyingbone segments;

FIG. 6E is a view similar to FIG. 6D, but showing the second anchorsinserted and attached to the first anchors, and further attached to theunderlying bone segments;

FIG. 7A is a perspective view of an anchor-in-anchor system constructedin accordance with another embodiment and including a first bone anchorreceiving and attached to a second bone anchor;

FIG. 7B is a sectional side elevation view of the anchor-in-anchorsystem illustrated in FIG. 7A;

FIG. 7C is an exploded view of the anchor-in-anchor system illustratedin FIG. 7A;

FIG. 8A is a side elevation view of the anchor-in-anchor systemillustrated in FIG. 7A attached to underlying bone;

FIG. 8B is an exploded view showing the first anchor illustrated in FIG.7A attached to a bone plate, and the second anchor illustrated in FIG.7A aligned for insertion into the first anchor;

FIG. 8C is an exploded view similar to FIG. 8B, but showing the secondanchor inserted in, and attached to, the first anchor;

FIG. 9A is a side elevation view of the anchor-in-anchor systemillustrated in FIG. 7A constructed in accordance with an alternativeembodiment;

FIG. 9B is a sectional side elevation view of a bone plate incorporatingthe anchor-in-anchor system illustrated in FIG. 9A;

FIG. 10A is a side elevation view of the anchor-in-anchor systemillustrated in FIG. 7A constructed in accordance with anotheralternative embodiment;

FIG. 10B is a side elevation view of the anchor-in-anchor systemillustrated in FIG. 1 constructed in accordance with an alternativeembodiment;

FIGS. 11A-C are perspective views of anchor-in-anchor fixationassemblies constructed in accordance with alternative embodiments;

FIG. 12 is a perspective view of an anchor-in-anchor fixation assemblyincluding a plurality of anchor-in-anchor systems attached to a t-shapedbone plate;

FIG. 13A is a side elevation view of an intramedullary rod constructedin accordance with one embodiment;

FIG. 13B is a side elevation view of an anchor-in-anchor fixationassembly including an anchor-in-anchor system attached to theintramedullary rod;

FIG. 13C is a side elevation view of the anchor-in-anchor fixationassembly including a plurality of anchor-in-anchor systems attached tothe intramedullary rod and further attached to a long bone;

FIG. 14A is a side elevation view of an anchor fixation assemblyincluding a plurality of anchor-in-anchor systems attached to anintramedullary rod in accordance with an alternative embodiment;

FIG. 14B is a side elevation view of the anchor fixation assemblyillustrated in FIG. 14, but including a plate attached toanchor-in-anchor systems;

FIG. 14C is a side elevation view of an anchor fixation assemblyincluding a plurality of anchor-in-anchor systems attached to anintramedullary rod in accordance with another alternative embodiment;

FIG. 15A is an end view of an anchor fixation assembly including a pairof anchor-in-anchor systems attached to a nail with one embodiment;

FIG. 15B is a side elevation view of the anchor fixation assemblyillustrated in FIG. 15A;

FIG. 16A is an exploded perspective view of an anchor-in-anchor fixationassembly including an expandable bone plate;

FIG. 16B is a bottom perspective view of the anchor-in-anchor fixationassembly illustrated in FIG. 16A;

FIG. 16C is a top perspective view o the anchor-in-anchor fixationassembly illustrated in FIG. 16A;

FIG. 17A is a top perspective view of the anchor-in-anchor fixationassembly illustrated in FIGS. 16A-C further including an intervertebralimplant;

FIG. 17B is a bottom perspective view of the anchor-in-anchor fixationassembly illustrated in FIG. 17A;

FIG. 17C is a perspective view of the anchor-in-anchor fixation assemblyillustrated in FIG. 17B implanted in a spine;

FIG. 17D is a schematic top view of the anchor-in-anchor fixationassembly as illustrated in FIG. 17C;

FIG. 18 is a perspective view of an anchor-in-anchor fixation assemblyincluding an eccentric bone plate;

FIG. 19A is a sectional side elevation view of an anchor-in-anchorfixation assembly including a pedicle screw assembly;

FIG. 19B is a perspective view of the anchor-in-anchor fixation assemblyillustrated in FIG. 19A implanted into a spine;

FIG. 19C is a side elevation view of the anchor-in-anchor fixationassembly illustrated in FIG. 19A implanted into a spine;

FIG. 20A is an exploded perspective view of an anchor-in-anchor fixationassembly including a shoulder prosthetic;

FIG. 20B is a sectional side elevation view of the anchor-in-anchorfixation assembly illustrated in FIG. 20A; and

FIG. 20C is a perspective view of the assembled anchor fixation assemblyillustrated in FIG. 20A.

DETAILED DESCRIPTION

Certain terminology is used in the following description for convenienceonly and is not limiting. The words “right,” “left,” “top,” and “bottom”designate in the drawings to which reference is made. The words“inwardly” and “outwardly” refer to directions toward and away from,respectively, the geometric center of the device and designated partsthereof. The words “anterior,” “posterior,” “superior,” “inferior,” andrelated words and/or phrases designate preferred positions andorientations in the human body to which reference is made and are notmeant to be limiting. The terminology includes the above-listed words,derivatives thereof and words of similar import.

With initial reference to FIG. 1, an anchor-in-anchor fixation system 20is illustrated as attached to an underlying structure or bone 21. Thefixation system 20 is illustrated as a bone fixation system inaccordance with one example embodiment that includes a first or primarybone fixation element or bone anchor 22 and a second or auxiliary bonefixation element or bone anchor 24 that is received by the first boneanchor 22. As will become appreciated from the description below, thebone fixation system 20 can be used to securely fasten auxiliaryfixation devices such as external fixators, internal bone fixationdevices, spinal fixation devices, and the like, to underlying bone.Unless otherwise indicated, the bone fixation system 20 and itscomponents can be manufactured from any suitable biocompatible materialknown in the art including but not limited to titanium, titanium alloysuch as TAN, stainless steel, reinforced plastics, allograft bone, andthe like, unless otherwise indicated.

Referring also to FIGS. 2A-C, the first bone anchor 22 includes a shaft26 that extends longitudinally along a central longitudinal axis L1. Thebone anchor 22 includes a shaft 26 that defines longitudinally opposingproximal, or upper, and distal, or lower, ends 26 a and 26 b,respectively, and a head 28 coupled to the proximal end 26 a. Helicalthreads 30 extend radially out from the shaft 26 at locations at andbetween the proximal and distal ends 26 a-b that are configured toengage underlying bone. Thus, a substantial entirety of the shaft 26 canbe threaded. As illustrated in FIG. 1, the threads 30 define an outerdiameter OD1 that increases in a direction from the distal end 26 btoward the proximal end 26 a. Accordingly, the threads 30 disposed atthe proximal end 26 a define an outer diameter that is greater than theouter diameter of the threads 30 disposed at the distal end 26 b.Alternatively, as illustrated in FIGS. 2A-B, the outer diameter OD1 ofthe threads 30 is constant across the proximal and distal ends 26 a-b.It should thus be appreciated that the first bone anchor 22 can provideas a locking screw as illustrated, though it should be appreciated thatthe first bone anchor can alternatively be provided as a compressionscrew, a nail, rivet, or pin whose shaft is smooth or ribbed, asdesired.

The head 28 includes an annular body 32 that defines a radially innersurface 33, an opposing radially outer surface 35, a proximal, or upper,end 28 a and a distal, or lower, end 28 b. The annular body 32 candefine the shape of a segment of a sphere as illustrated, having adiameter or cross-sectional dimension that is greater at a locationbetween the proximal and distal ends 28 a-b than at either of theproximal and distal ends 28 a-b. Accordingly, the radially outer surface35 can be spherical or otherwise convex. Of course, the head 28 canassume any other suitable alternative shape as desired.

The distal end 28 b of the head 28 is coupled to the proximal end 26 aof the shaft 26, either directly or indirectly via an unthreaded neck 34that is coupled between the proximal end 26 a of the shaft 26 and thedistal end 28 b of the head 28. The annular body 32 can include a base37 at the distal end 28 b that extends continuously in a circumferentialdirection about the head 28. The annular body 32 further includes aplurality of circumferentially spaced retention tabs 36 that extend upfrom the distal end 28 b. Thus, the terminal ends of the retention tabs36 are disposed at the proximal end 28 a of the head 28. The retentiontabs 36 are configured such that circumferentially adjacent tabs 36 areseparated by a slot 38 that extends distally into the proximal end 28 aof the head 28 in a direction toward, but not through, the base 37.

The head 28 further defines a bore 40 extending centrally through theannular body 32 along a central bore axis C1. The central axis C1extends in a direction angularly offset with respect to the longitudinalaxis L1. The shaft 26 is coupled to the base 37, and extends radiallyoutward and down from the radially outer surface 35 of the base 37 suchthat the shaft 26 does not interfere with the bore 40. The head 28includes a plurality of helical threads 41 in the bore 40 that extendradially inward from the radially inner surface 33 of the annular body32, including the tabs 36 and the base portion 37. In the illustratedembodiment, the central axis C1 of the bore 40 intersects with thelongitudinal axis L1 of the shaft 26 so as to define an acute angle α.The angle α is illustrated as being acute. Thus, in accordance with oneembodiment, the angle is between 0° and 90°, for instance between 20°and 60°, for instance between 20° and 40°.

In the illustrated embodiment, the central axis C1 is normal withrespect to the proximal and distal ends 28 a-b, though it should beappreciated that the proximal and distal ends 28 a-b could be configuredsuch that the central axis C1 defines a non-perpendicular angle withrespect to one or both of the proximal and distal ends 28 a-b.Therefore, with continuing reference to FIGS. 1-2B, the head 28 of thefirst bone anchor 22 defines a central head axis D1 defined by theproximal and distal ends 28 a-b. In particular, central axis D1 extendsin a direction normal with respect to the proximal and distal ends 28a-b. Because the central axis C1 of the bore 40 extends parallel to thedirection extending between the proximal and distal ends 28 a-b in theillustrated embodiment, the axes C1 and D1 are longitudinally alignedand substantially coincident, and thus define the same angle α withrespect to the longitudinal axis L1 of the shaft 26. Otherwise stated,the orientation of the head 28 is angularly offset with respect to theshaft 26 equal to the angular offset of the bore 40 with respect to theshaft 26, though it should be appreciated that the axes C1 and D1 mayalternatively be angularly offset as desired.

While the inner surface 33 can include threads 41 extendingcircumferentially continuously within the bore 40 as illustrated inFIGS. 2A-B, it is appreciated that the head 28 can be constructed inaccordance with an alternative embodiment that allows the second boneanchor 24 to attached inside the head 28 at variable angles with respectto the central bore axis C1, head axis D1, and longitudinal axis L1. Inparticular, as illustrated in FIG. 2C, the head 28 can include aplurality of recesses 51 extending into the inner surface 33. Therecesses 51 can define a portion of a cylinder, and thus configured toreceive a corresponding portion of the second bone anchor 24. Thethreads 41 extend between adjacent recesses 51. While four recesses 51are shown as being circumferentially equidistantly spaced at 90° withrespect to each other, it should be appreciated that the head 28 caninclude any number of recesses 51 so as to allow the second bone anchor24 to be inserted into the head 28 at any desired angle, as will bedescribed below with reference to FIG. 4D.

Referring now to FIGS. 1 and 3, the second bone anchor 24 includes ashaft 42 that extends longitudinally along a central longitudinal axisL2. The shaft 42 can be longer, shorter, or substantially equal to thelongitudinal length of the shaft 26. The shaft 42 defines longitudinallyopposing proximal, or upper, and distal, or lower, ends 42 a and 42 b,respectively. The bone anchor 24 includes a head 44 coupled to theproximal end 42 a of the shaft 42. Helical threads 46 extend radiallyout from the shaft 42 at locations at and between the proximal anddistal ends 42 a-b that are configured to engage underlying bone. Thus,the substantial entirety of the shaft 42 can be threaded. The threads 46define an outer diameter OD2 that is constant across the proximal anddistal ends 42 a-b as illustrated, though the threads 46 canalternatively increases in a direction from the distal end 42 b towardthe proximal end 42 a as described above with respect to the shaft ofthe bone anchor 22. The outer diameter OD2 can be greater than, lessthan, or substantially equal to the outer diameter OD1. The threads 46can define the same pitch or a different pitch with respect to thethreads 30 of the first bone anchor 22.

The head 44 includes an annular body 48 that defines a radially innersurface 43 an opposing radially outer surface 45, a proximal, or upper,end 44 a and a distal, or lower, end 44 b. The outer surface 45 extendsconcentrically about an axis C2, and can define the shape of a frustumas illustrated having an outer diameter OD3 or cross-sectional dimensionthat increases in a direction from the distal end 44 b of the head 44toward the proximal end 44 a. Alternatively, the head can assume anysuitable alternative shape as desired, such as a segment of a sphere asillustrated, having a diameter or cross-sectional dimension that isgreater at a location between the proximal and distal ends 44 a-b thanat either of the proximal and distal ends 44 a-b. In the illustratedembodiment, the central axis C2 is parallel and coincident or alignedwith the longitudinal axis L2 of the shaft 42, though it should beappreciated that the central axis C2 could be angularly offset from thelongitudinal axis L2 if desired.

The distal end 44 b of the head 44 is coupled to the proximal end 42 aof the shaft 42, either directly as illustrated, or indirectly via anunthreaded neck 39 of the type described above with respect to the firstbone anchor 22. The head 44 includes helical threads 50 that extendradially out from the outer surface 45 of the annular body 48. It shouldthus be appreciated that the second bone anchor 24 can provide as alocking screw as illustrated, though it should be appreciated that thesecond bone anchor can alternatively be provided as a compression screw,a nail, rivet, or pin whose shaft is smooth or ribbed, as desired.

The head 44 further defines a central axis D2 defined by the proximaland distal ends 44 a-b. In particular, the central axis D2 extends in adirection normal with respect to the proximal and distal ends 44 a-b.Because the central axis C2 head 44 extends parallel to the directionextending between the proximal and distal ends 44 a-b in the illustratedembodiment, the axes C2 and D2 are coincident or aligned, and thusextend parallel and coincident or aligned with the longitudinal L2 inthe illustrated embodiment. Of course, it should be appreciated that theproximal and distal ends 44 a-b could be geometrically configured suchthat the axes C2 and D2 are angularly offset from each other.

The threads 50 define an outer diameter OD3 that increases in adirection from the distal end 44 b of the head 44 to the proximal end 44a of the head 44. Thus, the outer diameter of the threads 50 is greaterat the proximal end 44 a than at the distal end 44. The inner diameterof the threads 41 of the bore 40 can define an inner diameter thatincreases in a direction from the distal end 44 b toward the proximalend 44 a, such that the threads 50 and 41 are configured to mate. Itshould be appreciated, of course, that the outer diameter of the threads50 and 41 could be constant across the corresponding proximal and distalends.

The assembly of the bone fixation system will now be described withrespect to FIGS. 1 and 4A-C. In particular, the bore 40 of the firstbone anchor 22 is configured to receive the second bone anchor 24, suchthat the first and second bone anchors 22 and 24 are fastened together.Thus, during use, the surgeon forms an incision to access a targetedarea of the underlying bone 21. Next, the first bone anchor 22 isdriven, for instance screwed, into the underlying bone 21 such that thethreads 30 engage and attach the bone anchor 22 to a select one of theunderlying bone segments 21 a-b. Either or both of the bone anchors 22and 24 can be self-tapping, and thus include cutting flutes 25, or abore can be pre-drilled into the bone 21 prior to insertion of the shaft26 into the bone 21. The first bone anchor 22 is inserted into theunderlying segment such as a bone segment 21 a to a sufficient depth,and is rotated until the axis C1 of the bore 40 is aligned with adesired fixation location of a second underlying segment such as asecond bone segment 21 b. Once the first bone anchor 22 has beenfastened to the underlying bone, the second bone anchor 24 is insertedinto the underlying bone 21 through the head 28 of the first bone anchor22. The second bone anchor 24 can be inserted into the underlying bone21 through the same incision that received the first bone anchor 22, orthrough a second incision as desired.

In one embodiment, the first bone anchor 22 is fastened to a firstfractured segment 21 a of the underlying bone 21, which can be a longbone, such as a femur, humorous, tibia, radius, ulna, or any other boneas desired, and the second bone anchor 24 is fastened to secondfractured segment 21 b of the underlying bone 21. For instance, thefirst bone anchor 22 can be fastened to the shaft or intramedullaryportion of a patient's femur, while the second bone anchor 24 can befastened to the head portion of the patient's femur. In this regard, theanchor-in-anchor bone fixation system 20 can be used to fix a femoralfracture, though it should be understood that the fixation system 20 andsurgical methods of using the fixation system is equally applicable inother surgical procedures in which a surgeon desires to couple one ormore bones or bone fragments.

With continuing reference to FIGS. 1 and 4A-C, the outer diameter OD2 ofthe threads 46 is less than the inner diameter of the bore 40 extendingthrough the head 28 of the first bone anchor 22, such that the shaft 42can be driven linearly down through the bore 40 as shown in FIG. 4B.Alternatively, the outer diameter OD2 can be sized such that the threads46 can engage the threads 41 of the head 28 as the bone anchor 24 isrotated therein. As described above, the outer threads 50 of the head 44are configured to mate with the inner threads 41 of the head 28.Accordingly, the shaft 42 of the first bone anchor 22 can be driventhrough the bore 40 until either the distal end 42 b of the shaft 42engages the bone 21, or the threads 41 and 50 engage.

The bone anchors 22 and 24 can then be rotated with respect to eachother to longitudinally advance the shaft 42 into the bone 21, and tolongitudinally advance the head 44 inside the head 28 of the first boneanchor 22 at the same rate as the advancement of the shaft 42 into thebone, such that the first and second bone anchors 22 and 24 are lockedtogether. Thus, the second bone anchor 24 can attach to the first boneanchor 22 without compressing the first bone anchor 22 against theunderlying bone 21. Alternatively, the head 44 can be devoid of threads,and the inner surface 33 of the head 28 can be devoid of threads, suchthat the engagement of the head 44 against the head 28 causes the firstbone anchor 22 to compress against the underlying bone 22. The radiallyinner surface 43 of the head 44 can define a hexagonal or anyalternatively shaped structure that can be engaged by a screw drivinginstrument to rotate the head 44 inside the head 28. Alternatively oradditionally, a tool can be inserted into the slots 38 disposed betweenthe retention tabs 36 to prevent the first bone anchor 22 from rotatingalong with the second bone anchor 24.

Generally, the second bone anchor 24 engages the underlying bone 21prior to engagement of the threads 41 and 50. As the distal end 44 b ofthe head 44 of the second bone anchor 24 engages the proximal end 28 aof the head 28 of the first bone anchor 22, the retention tabs 36 canflex radially outward. Once the first and second bone anchors 22 and 24have been fully mated, the head 44 of the second bone anchor 24 isnested inside the head 28 of the first bone anchor 22, and the proximalend 44 a of the head 44 is substantially flush with the proximal end 28a of the head 28. The shaft 26 of the first bone anchor 22 extendsoblique with respect to the underlying bone 21, while the shaft 42 ofthe second bone anchor extends substantially normal with respect to theunderlying bone 21, though it should be appreciated that both shaftscould alternatively extend in a direction oblique to the underlyingbone.

The retention tabs 36 have particular utility when locking the head 28in an aperture of an auxiliary bone fixation device, such as a boneplate, an intramedullary nail or screw, an intervertebral implant, apedicle screw, or the like as will be described in more detail below. Itshould be appreciated that the head 28 can alternatively becircumferentially continuous at and between the proximal and distal ends28 a-b of the head.

In this manner, when locked inside one another, the bone anchors form astable triangular load bearing plane P defined by the longitudinal axesL1 and L2, and a direction extending between the shafts 24 and 42 (forinstance, between the terminal ends of the shafts 24 and 42). Thetriangular load bearing plane P is better able to withstand higherforces and prevent subsidence or migration of the bone anchor within thebone. That is, because the bone anchors 22 and 24 are angled withrespect to each other, each bone anchor resists migration within thebone due, for instance, to longitudinal forces applied to the other boneanchor that would tend to pull the bone anchor out of the underlyingbone 21. In this manner, the anchor-in-anchor bone fixation system 20enables a smaller auxiliary fixation device such as, for example, aplate, an intramedullary nail or screw, an intervertebral implant, orthe like, to be used while enabling the surgeon to insert a largernumber of bone anchors, such as bone anchors 22 and 24, to withstandanticipated loads.

Referring now to FIG. 4D, the head 28 illustrated in FIG. 2C allows thesecond bone anchor to attach to the head 28 such that the secondlongitudinal axis L2 defines variable angles with respect to the centralbore axis C1, the head axis D1, and the first longitudinal axis L1. Inparticular, the second bone anchor 24 can be inserted into the bore 40such that a first portion of the bone anchor 24 is disposed in one ofthe recesses, and a second portion of the bone anchor that is distalwith respect to the first portion is disposed in another one of the arecesses that is opposed to the recess through which the first portionof the bone anchor 24 is disposed. In the illustrated embodiment, thehead 44 is inserted into the bore 40 such that at a portion the proximalend of the head 44 is disposed in one of the recesses such that the headaxis D2, and thus the longitudinal axis L2, is angularly offset withrespect to the central bore axis C1 in a direction toward another one ofthe a recesses that is opposed to the recess through which the proximalend of the head 44 is disposed. It should be appreciated that the head44 can be disposed in any one of the recesses 51 as desired, such thatany one or all of the central axis C2, the head axis D2, and thelongitudinal axis L2 defines an angle, for instance between 0° and 30°,with respect to any one or both of the central axis C1 and the head axisD1.

The thread pitch of the threads 50 can vary fromnarrow-to-wide-to-narrow as measured along the central axis of theanchor 24 from one end (e.g., the proximal end) to the other end (e.g.,the distal end). This thread profile allows the anchor 24 to engage thebore 40 at a selectable angle within a range of angles while maintainingthe same degree of contact with the inner threads 41 regardless of theangle chosen, as described in U.S. patent application Ser. No.11/971,358, filed Jan. 9, 2009, the disclosure of which is herebyincorporated by reference as if set forth it its entirety herein. Thatis, the angle of the anchor 24 with respect to the central bore axis C1within the permissible range of angles does not affect the engagement ofthe threads 50 with the threads 41.

As will be described below, anchor-in-anchor bone fixation systems canbe used in long bone applications, shoulder prosthesis, spinalapplications, and can be used for stand-alone fixation whereby the boneanchors directly affix underlying bone segments, or can include one ormore auxiliary fixation devices such as bone plates, intramedullarynails or screws, intervertebral implants or other spinal implants suchas pedicle screws, and shoulder prosthesis. The anchor-in-anchorfixation systems of the type described herein can thus be used in longbone fracture fixation to fix two or more bones or segments, can be usedin the spine in a facet or laminoplasty fixation procedure, and shoulderprosthesis. It should be noted that it is not intended for any of theanchor-in-anchor systems as described herein to be limited to theparticularly identified procedures and/or applications unlessspecifically noted.

For instance, referring now to FIGS. 5 and 6A-F, one or moreanchor-in-anchor bone fixation systems 20 can be used in combinationwith an auxiliary fixation device such as a bone plate 52 so as todefine an anchor-in-anchor fixation assembly 23 configured to fix of oneor more long bones or bone fragments in a patient's body. Thus, thefixation assembly 23 includes at least one, such as a plurality ofanchor-in-anchor bone fixation systems, configured to be coupled to anauxiliary fixation device. While various embodiments of the fixationassembly 23 are illustrated with respect to one or both of the fixationsystems 20 and 120, it should be appreciated that either or bothfixation systems 20 or 120 could be coupled to the auxiliary fixationdevice unless otherwise indicated. The fixation assembly and itscomponents can be manufactured from any suitable biocompatible materialknown in the art including but not limited to titanium, titanium alloysuch as TAN, stainless steel, reinforced plastics, allograft bone, andthe like, unless otherwise indicated.

The bone plate 52 can be configured as desired, and includes an elongateplanar plate body 54 defining an inner bone-facing surface 53 and anopposing outer surface 55. One or more, such as a plurality, of bonefixation apertures 56 (a pair of apertures 56 as illustrated) extendsthrough the plate body 54 along a central axis A, which extendsperpendicular to the inner and outer surfaces 53 and 55 of the platebody 54. The plate body 54 thus defines a central plate portion 57 thatis disposed between the apertures 56 and configured to overlay afracture F that separates the bone segments 21 a-b. The plate body 54can be planar as illustrated, though it could be curved or shaped asdesired so as to conform partially or fully to the underlying bone,depending on the type of bone fixation being performed.

The apertures 56 present a spherical or otherwise convex inner surface58 that matches the contour of the outer radial surface 35 of the head28. A plurality of first bone anchors 22 is installed in the bone plate52 such that each head 28 is disposed in a corresponding aperture 56,the central axis C1 of the bore 40 coincides with the central axis A ofthe corresponding aperture 56, and the longitudinal axis L1 of the shaft26 extends down from the plate 52 in a direction that defines the angleα with respect to the axis A of the aperture 56. The second bone anchor24 is fastened to the first bone anchor 22 in the manner describedabove, such that the shaft 42 extends down from the plate 52 along theaxis A.

The method of attaching the bone plate 52 to underlying bone segments 21a-b will now be described with reference to FIGS. 6A-E. In particular, afirst bone anchor 22 is inserted into the bone segment 21 a, and thesecond bone anchor 24 is inserted into the bone segment 21 b. The bonesegments 21 a-b can be disposed on opposing sides of a fracture F. Thefirst bone anchors 22 can be inserted into the bone segments 21 a-b atan angle such that the head 28 is configured for insertion into theaperture 56 of the bone plate 52. In accordance with one embodiment, thebone anchor 22 is rotated in one of the underlying bone segments 21 a-buntil the central axis C1 of the bore 40 is oriented substantiallyvertically, or substantially normal to the surface of the bone segmentto which the second bone anchor is to be inserted. The second boneanchor 24 can be inserted into the same bone segment as the first boneanchor 22, or a different bone segment that is separated from the bonesegment of the first bone anchor 22 by a fracture. The threads 30 can beself-tapping, such that the bone anchors 22 are directly inserted intothe underlying bone segments 21 a-b. Alternatively, a guide bore can bedrilled into the underlying bone segments 21 a-b at a desired angularorientation, and the bone anchors 22 inserted into the pre-drilled guidebores.

Once the bone anchors 22 have been inserted into the underlying bonesegments 21 a-b at a desired depth as illustrated in FIG. 6A, thesurgeon may operatively couple the bone plate 52 to the pre-insertedfirst bone anchors 22. In particular, the bone plate 52 is placedover-top of the bone anchors 22 as illustrated in FIG. 6B, and broughtdown onto the bone anchors 22 such that the apertures 56 receive thecorresponding heads 28. The retention tabs 36 compress radially inwarduntil the mating surfaces of the head 28 and the aperture 56 arealigned, thereby causing the plate 52 to be “snapped” or “clicked” ontothe head 28 of each bone anchor 22 after the bone anchors 22 have beenaffixed to underlying bone as shown in FIG. 6C. In this regard, thesurgeon is provided with tactile feedback once the bone plate 52 hasbeen mated with the bone anchor heads 28.

Once the heads 28 have been disposed in the apertures 56, the sphericalor convex outer surfaces 35 of the heads 28 and the mating inner surfaceof the aperture 56 allows the bone anchor 22 to polyaxially rotate withrespect to the bone plate 52. Otherwise stated, the bone anchor 22 canbe inserted into the aperture 56 at any desired angular orientation solong as the second bone anchor 24 is able to pass through the bore 40 ofthe head 28 and into underlying bone. The first bone anchor 22 can beinserted into the underlying bone to provide compression of the plate 52against the bone segments if desired.

Next, referring to FIG. 6D, second bone anchors 24 are inserted into thehead 28 of a respective first bone anchor 22 and into the respectiveunderlying bone segments 21 a-b. In particular, the shaft 42 of thefirst bone anchor 22 is driven linearly through the bore 40 until eitherthe distal end 42 b of the shaft 42 engages the bone 21, or the threads41 and 50 engage. It should be appreciated that the shaft 42 of thesecond bone anchor 24 can extend in a direction substantiallyperpendicular to the bone plate 52. Typically the shaft 42 engages theunderlying bone 21 before the threads 41 and 50 engage. The verticalthickness of the plate body 54 is less than the vertical height of thehead 28, such that the proximal end 26 a of the shaft 26 does notinterfere with the plate 52 when the plate 52 is affixed to the heads28.

The bone anchor 24 can then be rotated to longitudinally advance theshaft 42 of the second bone anchor 24 into the bone 21, and tolongitudinally advance the head 44 inside the head 28 of the first boneanchor 22, such that the threads 50 extending in from the head 28 matewith the threads 41 extending out from the head 44 until the first andsecond bone anchors 22 and 24 are locked together as illustrated in FIG.6E. Thus, the shaft 26 of the first bone anchor 22 extends oblique withrespect to the bone plate 52 and underlying bone 21, and the shaft 42 ofthe second bone anchor 24 extends perpendicular with respect to the boneplate 52 and underlying bone 21, though it should be appreciated thatboth shafts 26 and 42 could alternatively extend in a direction obliqueto the bone plate 52 (and the underlying bone 21).

As the distal end 44 b of the head 44 of the second bone anchor 24engages the proximal end 28 a of the head 28 of the first bone anchor22, the retention tabs 36 flex radially outward against the innersurface 58 of the aperture 56, thereby causing a frictional fit thatsecures the position of the first bone anchor 22 with respect to theplate. Otherwise stated, the head 28 expands against the bone plate 52as the second bone anchor 24 mates with the first bone anchor 22. Inparticular, the outer surface 45 of the head 44 can taper radiallyoutward in a direction from the distal end 44 b toward the proximal end44 a. Accordingly, insertion of the second bone anchor 24 into andthrough the bore 40 radially expands the head 28 of the first boneanchor 22 against the inner surface 58 of the aperture 56.

Furthermore, as described above, both the radially inner surface 33 ofthe bone anchor head 28 and the radially outer surface 45 of the boneanchor head 44 are threaded so that the bone anchors 22 and 24 mate witheach other when the first bone anchor 22 receives the second bone anchor24, thereby securing the second bone anchor 24 to both the first boneanchor and furthermore to the bone plate 52. Thus, in use, theanchor-in-anchor bone fixation system 20 includes a first bone anchor 22having an expandable head 28 and a locking compression screw mechanismto lock a non-parallel second bone anchor in a bone plate 52.Accordingly, the bone fixation system 20 allows the placing of twonon-parallel bone anchors in a relatively small plate area so that ahigher level of stability can be achieved with respect to a bone platethat receives a single bone anchor in each aperture to affix the boneplate to underlying bone segments. Because one of the bone anchors (thefirst bone anchor 22 as illustrated) is angularly offset with respect tothe vertical direction of the second bone anchor 24, longitudinal forcesapplied to the second bone anchor that might otherwise cause migrationin the bone 21 are translated to the shaft 26 of the first bone anchor22, which resists the longitudinal forces that would tend to pull thebone anchor 24 out of the underlying bone 21.

Alternatively, the bone plate 52 can be placed against the bone segments21 a-b, and the shafts 26 can be inserted through the apertures 56 priorto affixing the shafts into the underlying bone. In this alternativeembodiment, the bone anchors 20 are inserted into the underlying bone toa desired depth, and the plate 52 is brought up against the heads 28. Ifthe slots 38 separating the retention tabs 36 extend significantly intothe distal portion 28 b of the heads 28, then the tabs 36 can compressradially inward as the plate is brought up over the heads 28.Alternatively the head 28 can include a single slot 38 that extendsvertically through the head 28 so as to define a pair of tabs 36 thatare separated at only one circumferentially outer end (see slot 138 inFIGS. 7-8). Alternatively, the outer surface 35 of the head 28 of thefirst bone anchor 22 can extend substantially linearly, for instancelongitudinally, such that the aperture 56 fits easily onto the head 28.Once the head 44 is fixed inside the head 28, the radially outer surface45 expands radially outward against the bone plate 52 in the mannerdescribed above, thereby securely fastening the bone anchors 22 and 24to the bone plate 52.

While the bone fixation system 20 and the bone fixation assembly 23 havebeen illustrated and described in accordance with the first and secondbone anchors 22 and 24 constructed in accordance with one embodiment, itis envisioned that bone fixation systems and assemblies can be providedin accordance with numerous alternative embodiments whereby a secondbone anchor extends through a bore formed in the head of a first bone,such that the first and second bone anchors join to an underlyingstructure, segments of an underlying structure, or different structures.

For instance, referring now to FIGS. 7A-C, an anchor-in-anchor fixationsystem 120 is illustrated whereby reference numerals corresponding tolike elements of the fixation system 20 described above are incrementedby 100. Thus, the fixation system 120 includes a first or primary boneanchor 122 and a second or auxiliary bone anchor 124 that is received inthe head 128 of the first bone anchor 122. The second bone anchor 124 isconstructed as described above with respect to the second bone anchor24, while the first bone anchor 122 is constructed such that the head128 is orientated parallel with respect to the shaft 126.

In particular, the proximal end 126 a of the shaft 126 is attached tothe distal end 128 b of the head 128, such that the shaft 126 iscentrally disposed with respect to the head 128 and extendslongitudinally down from the head 128. Thus, the central axis D1′ thatextends normal with respect to the proximal and distal ends 128 a-b isparallel and coincides with the longitudinal axis L1 of the shaft 26. Itshould be appreciated, of course, that the shaft 126 could be offsetfrom the central axis D1 of the head 128. The bore 140 extends throughthe head 128 along a central axis C1 that is angularly offset withrespect to both the longitudinal axis L1 of the shaft 126, and thecentral axis D1 that extends normal with respect to the proximal anddistal ends 128 a-b of the head 128. In particular, the central axis C1of the bore 140 forms an acute angle α with respect to the longitudinalaxis L1 and the central axis D1′. Thus, the angle α is between 0° and90°, such as between 40° and 60°. The head 128 can include retentiontabs of the type described above with respect to the head 28 of the boneanchor 22, or the head 128 can be circumferentially continuous at andbetween the proximal and distal ends 128 a-b as illustrated in FIG. 7A.

Referring now also to FIG. 8A, the fixation system 120 can be used as astand-alone system to couple two or more bone fragments together, forexample, in a “butterfly” fracture fixation procedure of a long bone,which may have occurred as a result of traumatic loading. Asillustrated, the bore 140 of the first bone anchor 122 is configured toreceive the second bone anchor 124, such that the first and second boneanchors 122 and 124 are fastened together, and are also fastened tounderlying bone 21. Thus, during use, the surgeon forms an incision toaccess a targeted area of the underlying bone 21. Next, the first boneanchor 122 is driven, for instance screwed, into the underlying bone 21or bone segment 21 a, such that the threads 130 engage and attach thebone anchor to the underlying bone 21.

The first bone anchor 22 is inserted into the underlying segment such asa bone segment 21 a at a sufficient depth, and is rotated until the axisC1 of the bore 140 is aligned with a desired insertion location of asecond underlying segment such as a second bone segment 21 b. In thisregard, it should be appreciated that the first bone segment 122 can beinserted into the bone segment 21 a in a direction normal to the planedefined by the underlying bone 21, or can be inserted in a directionangularly offset with respect to a direction normal to the plane definedby the underlying bone 21.

Once the first bone anchor 122 has been fastened to the underlying bonesuch that the axis C1 is aligned with a target location for the secondbone anchor 124, the second bone anchor 124 is inserted into theunderlying bone 21, such as segment 21 b, through the head 128 of thefirst bone anchor 122. The second bone anchor 124 can be inserted intothe underlying bone 21 through the same incision that received the firstbone anchor 122, or through a second incision as desired. Thus, thefirst bone anchor 122 can be inserted, for instance, into a fragmentedlong bone segment 21 a, while the second bone anchor 124 can beinserted, for instance, into a non-fragmented long bone segment 21 b,thereby securing the fragmented bone segment 21 a to the non-fragmentedbone segment 21 b. As illustrated, the shafts 126 and 142 of the firstand second bone anchors 122 and 124 each extend oblique with respect tothe underlying bone 21, though one of the shafts could alternativelyextend substantially perpendicular with respect to the underlying bonein the manner described above.

While the fixation system 120 has been illustrated and described asbeing used to secure a free-floating bone fragment to a long bone, itshould be appreciated that the anchor-in-anchor fixation systems asdescribed herein can also be used to secure a bone fragment in otherparts of the body as well, including but not limited to the cranium,face, hands, feet, pelvis, and the like. The anchor-in-anchor fixationsystems of the type described herein can also be used to secure onefragment to another fragment (for instance one bone fragment to anotherbone fragment), or one structure to another structure (for instance onebone to another bone).

It will also be appreciated that anchor-in-anchor bone fixation systemsenable a smaller auxiliary fixation device such as, for example, aplate, an intramedullary nail or screw, an intervertebral implant, orthe like, to be used while enabling the surgeon to insert a largernumber of bone anchors, such as bone anchors 122 and 124, to withstandanticipated loads. Moreover, when locked inside one another, the boneanchors form a stable triangular load bearing plane P and are thusbetter able to withstand higher forces and prevent subsidence ormigration. That is, because the bone anchors 122 and 124 are angled withrespect to each other, each bone anchor resists migration within thebone due, for instance, to longitudinal forces applied to the other boneanchor that would tend to pull the bone anchor out of the underlyingbone 21.

Referring now to FIGS. 7A-C and 8B-C, the anchor-in-anchor fixationassembly 23 can alternatively or additionally include one or moreanchor-in-anchor bone fixation systems 120 in combination with anauxiliary fixation device such as a bone plate 152 configured forfixation of one or more long bones or bone fragments in a patient's bodygenerally in the manner described above with respect to the bonefixation system 20. In particular, a plurality of first bone anchors 122is installed in the bone plate 152 such that each head 128 is disposedin a corresponding aperture 156. When installed, the shafts 126 extenddown from the plate 152 such that the longitudinal axis L1 extends in adirection substantially parallel and coincident with the central axis Aof the corresponding aperture 156, and the central axis C1 of the bore140 defines the angle α with respect to the central axis A. It shouldthus be appreciated that the shafts 126 therefore extend in a directionsubstantially perpendicular to the bone plate 152, though it should beappreciated that the shafts 126 could alternatively define anon-perpendicular angle with respect to the bone plate 152.

Once the first bone anchor 122 has been positioned such that the centralaxis C1 is aligned with a target location of the second bone anchor 124,the second bone anchor 124 is fastened to the first bone anchor 122 inthe manner described above, such that the shaft 142 extends down fromthe plate 152 in a direction coincident with the central axis C1, so asto define the angle α with respect to the axis A. As illustrated inFIGS. 8B-C, a slot 138 extends vertically through the head 128 so as todefine a pair of retention tabs 136 separated by the slot 138.Accordingly, the tapered head 144 causes the head 128 to flex radiallyoutward against the inner surfaces of the apertures 156 so as to lockthe head 128 against the plate 152 in the manner described above. Theshaft 126 of the first bone anchor 122 thus extends perpendicular withrespect to the bone plate 152 and underlying bone 21, and the shaft 142of the second bone anchor 124 extends oblique with respect to the boneplate 152 and underlying bone 21, though it should be appreciated thatboth shafts 126 and 142 could extend oblique with respect to the boneplate 152 and underlying bone 21.

Alternatively, the bone plate 152 can be placed against the bonesegments 21 a-b, and the shafts 126 can be inserted through theapertures 156 prior to affixing the shafts into the underlying bone. Inthis alternative embodiment, the heads 128 are brought down into theapertures 156 of the bone plate 152. Once the head 144 is coupled to thehead 128, the radially outer surface 145 expands radially outwardagainst the bone plate 152 in the manner described above, therebysecurely fastening the bone anchors 122 and 124 to the bone plate 152.

Furthermore, while the bone anchors 22, 24, 122, and 124 have beendescribed in accordance with particular illustrated embodiments wherebythe threads extend along an entirety of the shaft, for instance at andbetween the proximal and distal ends of the shaft, the bone anchors usedin combination with any of the bone fixation systems described hereincan be alternatively constructed.

For instance, referring to FIGS. 9A-B, the first bone anchor 122 isillustrated as including a plurality of threads 130 that extend radiallyout from the shaft 126 along the full length of the shaft 126 in themanner described above, and also extend radially out from the head 128,thereby providing a locking screw. The threads 130 extending out fromthe head 128 are configured to engage complementary threads 183extending radially in from the inner surface 158 of the aperture 156 tofurther affix the first bone anchor 122 to the bone plate 152. Thus, thefirst bone anchor 122 is threadedly coupled to the bone plate 152, andthe second bone anchor 124 is threadedly coupled to the first boneanchor 122.

In this manner, the first bone anchor 122 is inserted into theunderlying bone and through the aperture 156, the shaft 126 is firstinserted through the apertures 156 such that the threads 130 extendingout from the head 128 engage the threads in the inner surface 158 tosecure the bone anchor 122 to the bone plate 152. In this regard, itshould be appreciated that the threaded engagement between the head 128and the bore 156 fixes the angular position of the first bone anchor 122relative to the bone plate 152 such that the second bone anchor head 144need not expand the head 128 against the inner surface 158 in the mannerdescribed above. Furthermore, the threaded engagement between the head128 and the bone plate 152 allows the bone anchors 122 and 124 to attachthe plate 152 to the underlying bone without compressing the plate 152against the underlying bone.

While the bone plate 52 has been illustrated as attached to underlyingbone 21 via the fixation systems 20 and the bone plate 152 has beenillustrated and described as attaching to underlying bone 21 via thefixation systems 120, it should be appreciated that one or more fixationsystems 20 and 120 could be used in combination to attach either of thebone plates 52 and 152 to the underlying bone 21.

Whether the fixation system 120 is provided as a stand-alone construct(i.e., without an auxiliary fixation device) or in combination with anauxiliary fixation device such as the bone plate 152 to provide afixation assembly 23, the threaded engagement between the second boneanchor 124 and the first bone anchor 122 allows the head 144 of thesecond bone anchor 124 to be countersunk within the head 128 of thefirst bone anchor 122, such that the proximal end 128 a of the head 128is substantially flush to provide a low profile and minimize the traumaassociated with the fixation system 120.

Alternatively, referring to FIG. 10A, the first bone anchor 122 can beprovided having threads 130 that extend only partially along the shaft126. As illustrated, the threads 130 extend radially out from the distalend 126 b of the shaft, such that the proximal end 126 a of the shaft126 is smooth and devoid of threads. It should be appreciated that thebone anchor 122 could alternatively include one or more fixation ribsextending radially out from the proximal end 126 a that engagesurrounding bone and resist migration of the bone anchor 122 within thebone. It should be further appreciated that the second bone anchor 124can likewise present threads 130 that extend only partially along theshaft 142, and in particular extend radially out form the distal end 142b of the shaft 142, such that the proximal end 142 a of the shaft isdevoid of threads. Threads could additionally or alternatively extendfrom the heads 128 and 144 in the manner described above.

Referring to FIG. 10B, the first bone anchor 22 can be provided havingthreads 30 that extend only partially along the shaft 26. Asillustrated, the threads 30 extend radially out from the distal end 26 bof the shaft 26, such that the proximal end 26 a of the shaft 26 issmooth and devoid of threads. It should be appreciated that the boneanchor 22 could alternatively include one or more fixation ribsextending radially out from the proximal end 26 a that engagesurrounding bone and resist migration of the bone anchor 22 within thebone. It should be further appreciated that the second bone anchor 24can likewise present threads 46 that extend only partially along theshaft 42, and in particular extend radially out form the distal end 42 bof the shaft 42, such that the proximal end 42 a of the shaft is devoidof threads. Threads could additionally or alternatively extend from theheads 28 and 44 in the manner described above.

It should alternatively be appreciated that one or more, up to all ofthe bone anchors 22, 24, 122, and 124 could be constructed having fullythreaded shafts, partially threaded shafts, or shafts that are entirelydevoid of threads. For instance, FIGS. 11A-C illustrate exampleembodiments of the fixation system 20 as including bone anchors 22 and24 whose shafts 26 and 42 are constructed in accordance with alternativeembodiments, though the bone anchors 122 and 124 can be constructed asillustrated and described with respect to the bone anchors 22 and 24.

For instance, referring to FIG. 11A the shaft 26 of the first boneanchor 22 is threaded along its entirety as described above with respectto FIG. 1, however the shaft 42 of the second bone anchor 24 is devoidof threads so as to define an unthreaded nail, rivet, or unthreaded pin.The use of such unthreaded bone anchors can be particularly useful inapplications where the fixation system 20 or 120 is being implementedfor fixation in cancellous bone. It should be appreciated that theunthreaded bone anchor 24 can include one or more locking teeth 47projecting radially out from the shaft 42. The teeth 47 can define ascrew helix pattern about the shaft 42, but also allow the bone anchor24 to be hammered into the underlying bone. The radially outer surface45 of the head 44 flares radially outward in a direction from the distalend 44 b toward the proximal end 44 a. Accordingly, once the first boneanchors 22 are attached to underlying bone in the manner describedabove, the shaft 42 of the second bone anchor 24 is inserted into thehead 28 of the first bone anchor, and subsequently hammered into theunderlying bone. As the head 44 is inserted into the head 28, the outersurface 45 causes the retention tabs 36 to flex radially outward so asto lock the fixation member 22 in the plate 52 as described above.

Referring now to FIG. 11B, the shaft 42 of the second bone anchor 24 isthreaded along its entirety as described above with respect to FIG. 1,however the shaft 26 of the first bone anchor 22 is devoid of threads soas to define an unthreaded nail, rivet, or unthreaded pin. The shaft 26can have a substantially constant diameter as illustrated in FIG. 11A,or the outer diameter can differ along the length of the shaft. Forinstance, as illustrated, the proximal end 26 a of the shaft 26 definesan outer diameter that is greater than the outer diameter at the distalend 26 b of the shaft 26 a. It should be appreciated that the unthreadedbone anchor 22 can include one or more locking teeth projecting radiallyout from the shaft 26. Accordingly, the first bone anchors 22 can behammered into the underlying bone, and the bone plate 52 can be attachedto the heads 28 of the bone anchors 22 in the manner described above.The second bone anchors 24 can then be inserted into the underlyingbone, and the heads 44 can be attached to the heads 28 of the first boneanchors 22 in the manner described above.

Alternatively still, as illustrated in FIG. 11C, the shafts 26 and 42 ofboth bone anchors 22 and 24 can be devoid of threads. One or both of theshafts 26 and 42 can present constant outer diameters along theirlengths, or can present different outer diameters along their lengths.For instance, the proximal ends 26 a of the shafts 26 present an outerdiameter greater than that of the distal ends 26 b of the shafts 26.During operation, the shaft 26 of the first bone anchor 22 can behammered into underlying bone, and the bone plate 52 can be attached tothe head in the manner described above. Subsequently, the shaft 42 ofthe second bone anchor 24 can be inserted through the head 28 and driveninto underlying bone until the head 44 nests within the head 28, in themanner described above with reference to FIG. 11A.

While the anchor-in-anchor bone fixation systems 20 and 120 have beendescribed in combination with an elongate plate configured to providelong bone fixation, it should be appreciated that the anchor-in-anchorbone fixation systems of the type described herein could alternativelyinclude bone plates of any suitable size and shape. For instance,referring to FIG. 12, a bone plate 252 is illustrated having referencenumerals corresponding to like elements of the bone plate 52 incrementedby 200. Therefore, the plate 252 includes a plate body 254 having afirst longitudinally elongate section 254 a that is configured to extendparallel to the underlying long bone, and a laterally elongate secondsection 254 b disposed at one end of the first elongate section 254 aand elongate in a direction perpendicular to the first elongate section254 a. Thus, the plate 252 is T-shaped such that the second elongatesection 254 b extends generally perpendicular to the underlying longbone. The plate sections 254 a-b can further be curved so as to conformto the long bone.

A plurality of longitudinally spaced apertures 256 a extends through thefirst plate section 254 a, and a plurality of laterally spaced apertures256 b extends through the second plate section 254 b. In one embodiment,it is envisioned that the T-shaped bone fixation plate is configured toattach to a tibia having a fracture that is disposed longitudinallybetween the plurality of apertures 256 a and the plurality of apertures256 b. Either or both of the fixation systems 20 and 120 can be insertedinto one or more, up to all, of the apertures 256 a-b. As illustrated,the fixation system 20 is inserted into the apertures 256 a-b in themanner described above with respect to bone plate 52. It can beappreciated that the shafts 26 of the first bone anchors 22 can bealigned in any angular direction as desired, and can be constructedsufficiently long so as to extend through overlaid bone segment, throughthe fracture, and into the opposing fractured bone segment.

Referring now to FIGS. 13A-C, the anchor-in-anchor fixation assembly 23can include one or both fixation systems 20 and 120 and an auxiliaryfixation device provided as intramedullary nail, screw, or rod 70(collectively referred to herein as a “rod”) for use in long bonefixation. As generally understood in the art, the intramedullary rod 70is configured to be inserted into the intramedullary canal of the longbone to be fixed. Referring in particular to FIG. 13A, theintramedullary rod 70 includes a longitudinally extending tubular rodbody 72 including a first segment 72 a and a second segment 72 bcoextensive with the first segment 72 a. The second segment 72 b isillustrated as a head portion of the rod body 72 that defines an outerdiameter greater than that of the first segment 72 a. The rod definesone or more first bone fixation apertures 74 a extending through thefirst rod segment 72 b, and one or more second bone fixation apertures74 b extending through the second rod segment 72 b.

The apertures 74 a can be longitudinally displaced from each other alongthe length of the rod segment 72 a, and can further be angularlydisplaced from each other about the circumference of the rod segment 72a. Likewise, the apertures 74 b can be longitudinally displaced alongthe length of the rod segment 72 b, and can further be angularlydisplaced from each other about the circumference of the rod segment 72b. The apertures 74 a-b can extend centrally through the rod bodysegments 72 a-b coextensive with the diameter of the segments 72 a-b, oroffset from the center of the segments 72 a-b coextensive with a chordof the segments 72 a-b. The apertures 74 a-b can also define alongitudinal directional component such that one end of a given apertureis longitudinally displaced with respect to the opposing end of theaperture. While the rod 70 is illustrated as substantially cylindricalas illustrated, it should be appreciated that the rod 70 could assumeany suitable alternative shape and size as desired.

As illustrated in FIG. 13B, the anchor-in-anchor bone fixation system120 is configured to attach the intramedullary rod 70 to surroundingbone. In particular, one of the bone anchors, for instance the firstbone anchor 122, is driven into a selected one of the apertures 74, forinstance using a screwing, drilling, hammering, or like mechanism toinsert the shaft 26 into the selected aperture 74. The bone anchor shaft126 can define a length so as to terminate inside the rod 70 or extendthrough the rod 72 without passing through the opposing bone surface, orcan define a length sufficient to pass through both the rod 72 and theopposing bone surface. Once the shaft 126 is inserted at a desiredrotational position such that the central axis C1 of the bore 140 isaligned with a target location on the bone for fixation, the second boneanchor 124 is attached to the first bone anchor 122 and directly to thebone that surrounds the intramedullary rod 72 without also attaching tothe intramedullary rod. It should be appreciated, of course, that thesecond bone anchor 124 could also attach to an aperture 74 extendingthrough the intramedullary rod 72 if desired.

In accordance with the illustrated embodiment, the shaft 142 is insertedthrough the bore 140 and driven into the surrounding bone 21, forinstance using a screwing, drilling, hammering, or like mechanism toinsert the shaft 142 into the surrounding bone 21. In this regard, itshould be appreciated that, as in all embodiments of the ofanchor-in-anchor bone fixation systems described herein unless otherwisenoted, either or both of the bone anchors 122 and 124 could be providedwith threaded heads, unthreaded heads, heads including locking tabs 136or heads that are circumferentially continuous at and between theproximal and distal ends, fully threaded shafts, partially threadedshafts, or unthreaded shafts defining a smooth radially outer surface orincluding teeth 47 projecting out from the radially outer surface.

While the first bone anchor 122 is configured to be attached to the rod70 and the second bone anchor 124 is configured to be attached to thesurrounding bone, it should be appreciated that the second bone anchor124 could alternatively be attached to the rod 70 in the mannerdescribed above, and the first bone anchor 122 could be attached to thesurrounding bone. In this alternative embodiment, the first bone anchor22 is driven into the bone 21 such that the central axis C1 of the bore140 is aligned with one of the apertures 74. The second bone anchor 124is then inserted through the bore 140 such that the shaft 142 extendsthrough and is coupled to the aligned aperture 74.

It should further be appreciated that while the second fixation system120 has been illustrated as attached to the intramedullary rod 70 andsurrounding bone 21 in the manner described above, the first fixationsystem 20 could alternatively or additionally attach to theintramedullary rod 70 and surrounding bone. For instance, the first boneanchor 22 could attach to one of the apertures 74 and the second boneanchor could attach to the surrounding bone 21 in the manner describedabove, or the first bone anchor 22 could attach to the surrounding bone21, and the first bone anchor could attach to one of the apertures 74 inthe manner described above.

Referring also now to FIG. 13C, the intramedullary rod 72 can beinserted into the canal 49 of a fractured long bone 21 that definesfracture segments 21 a and 21 b. The rod 72 is inserted into the canal49 such that the fracture F is disposed between the first apertures 74 aand the second apertures 74 b. It should be appreciated that one or moreof the fixation systems 120 can attached to the surrounding bone 21 andfurther attach to one or more, up to all, of the one apertures 74 a-b inaccordance with any of the embodiments described above. Alternatively oradditionally, one or more of the fixation system 120 can attached to thesurrounding bone 21 and further attach to one or more, up to all, of theone apertures 74 a-b in accordance with any of the embodiments describedabove.

Thus, the fixation systems 20 and 120 can be used alone or incombination to fasten the intramedullary rod 72 to the surrounding bone.It should be further appreciated in all embodiments described herein,unless otherwise, noted, that the fixation systems 20 and 120 can beused in combination when attaching directly to bone as a stand-aloneconstruct, or when affixing an auxiliary fixation device to anunderlying structure such as bone, bone-substitutes or bone-spacers,allografts, autografts, synthetic grafts, and metal or titanium grafts.It should be further appreciated that the bone anchors that attach tothe intramedullary rod 72 can be rotated to any desired position, suchthat the other bone anchor can extend into the surrounding bone at anyangular orientation as desired.

In the embodiments illustrated, in FIGS. 13A-C, bone anchors areattached directly to an intramedullary rod and the surrounding bone 21as stand-alone constructs, wherein the bone anchors are not joined byany additional auxiliary fixation device. However, it should beappreciated that the anchor-in-anchor fixation assembly 23 can furtherinclude a second auxiliary fixation device, such as a bone plate usablein combination with a first auxiliary fixation device, such as anintramedullary rod, as will now be described in more detail below withreference to FIGS. 14A-C.

Referring now to FIG. 14A, the anchor-in-anchor fixation assembly 23 isillustrated as including a plurality of bone fixation systems 20attached to an intramedullary rod 170 constructed in accordance with analternative embodiment whereby reference numerals corresponding to likeelements of the intramedullary rod described above are incremented by100. The intramedullary rod 170 includes a tubular rod body 172 thatdefines a substantially constant outer diameter along its length. Therod 172 defines one or more apertures 174 extending through the rod body172. The rod 172 is configured to be placed in the canal of a long bonesuch that the apertures 174 are separated by a fracture in the mannerdescribed above. As illustrated, the bone fixation system 20 is attachedto the apertures 174, and is configured to attach to the surroundingbone in the manner described above, though it should be appreciated thatthe bone fixation system 120 can be used in combination with the bonefixation system 20 or alone to attach the intramedullary rod 170 to thesurrounding bone in the manner described herein.

As illustrated in FIG. 14B, the anchor-in-anchor fixation assembly 23can include a second auxiliary bone fixation device in the form of abone plate 52. The bone plate can be constructed as desired, andincludes a plate body 54 and a plurality of apertures 56 extendingthrough the plate body 54 and configured to attach to the bone fixationsystem 20 in the manner described above. The bone plate 52 can be placedover the fracture that extends across the intramedullary rod such thatthe fracture is disposed between apertures 56 of the bone plate 52. Forinstance, the first bone anchor 22 is illustrated as attached to theintramedullary rod 170 in the manner described above with respect to theintramedullary rod 70. The head 128 of the anchor 122 includes retentiontabs that are inserted into one of the apertures 56 of the bone plate 52in the manner described above. Accordingly, the bone anchor 122 isattached to both the bone plate 52 and the intramedullary rod 70. Thesecond bone anchor 124 can attach directly into the bone 21 withoutattaching to the intramedullary rod 170 in the manner described above.

It should be appreciated, alternatively, that the head 128 of the firstbone anchor 122 can attach to the bone plate 52 in the manner describedabove, and the shaft 126 can attach to the bone 21 without passingthrough the intramedullary rod 170, and that the second bone anchor canpass through the head 128 and attach to one of the apertures 174 of theintramedullary rod 170. Alternatively or additionally, it should beappreciated that the bone fixation system 20 can be used in combinationwith the bone fixation system 120 or alone to attach the intramedullaryrod 170 to the bone plate 52 and the surrounding bone in the mannerdescribed herein. It should further be appreciated that theintramedullary rod 70, or any intramedullary rod constructed as desired,can be attached to surrounding bone by one or both of the fixationsystems 20 and 120 alone, or in combination with a second auxiliary bonefixation device such as a bone plate. The bone plates of the typedescribed herein can be provided with any thickness as desired. Forinstance, the bone plates can be constructed sufficiently thin so as toprovide an in-between member for suture attachment between a pair ofbone anchors provided as screws, for instance, in order to fasten andreposition soft tissue structures such as tendons, ligaments, andmuscles.

As illustrated in FIGS. 14A-B, it is appreciated that the apertures 174can extend through the rod body 172 such that the apertures are definedon all sides by the rod body 172. Alternatively or additionally,referring to FIG. 14C, the apertures 174 can be provided as notches thatextend into the rod body 172. As illustrated, the notches 174 aregenerally rectangular in cross-section and have a thicknesssubstantially equal to the outer diameter of the bone anchor shaft thatis inserted therein. The notches 174 extend laterally into the rod body172 and terminate at a depth that is substantially equal to or slightlygreater than the outer diameter of the bone anchor shaft 26 that isinserted therethrough. The notches 174 can thus provide a friction fitwith the inserted shafts, or can be threaded so as to threadedly engagethe inserted shafts.

Referring now to FIGS. 15A-B, the anchor-in-anchor fixation assembly 23includes a nail 270 illustrated as including reference numeralscorresponding to like elements of the intramedullary rod 70 incrementedby 200. The nail 270 is configured for fixation to a pair of bonefragments separated, for instance, by a fracture of the distal radius.The nail 270 includes a tubular or alternatively shaped nail body 272having a central portion 272 a that has an outer diameter greater thanthe remaining region 272 b of the nail body 272 that is disposed on bothlongitudinally outer sides of the central portion 272 a. One or morelongitudinally spaced apertures 274 extends into and through the centralportion 272 a of the nail body 272. Each aperture 274 is provided withan associated oblique aperture 274 a that extends from the aperture 274at a location inside the nail body 272 and through the outer surface ofthe nail body 272. The oblique aperture 274 a extends along a centralaxis B that defines an angle with respect to the central axis A of theassociated aperture 274 that is equal to the angle α defined between theshafts 26 and 42 of the bone anchors 22 and 24. Each aperture 274 can beprovided with a pair of oblique apertures 274 a so as to provide forfixation flexibility.

The fixation system 20 is installed into the nail 270 by first insertingthe bone anchor 22 into the aperture 274 such that the shaft 26 extendsinto the proximal end of the aperture 274 and is then directed throughthe distal end of the associated auxiliary aperture 274 a. The aperture274 can define a diameter that is substantially equal to or slightlygreater than the outer diameter of the anchor head 28, such that theanchor head is recessed within the aperture 274. It should beappreciated that any of the auxiliary fixation devices could includeapertures configured such that the received bone anchor heads arerecessed therein. Once the bone anchor 22 has been installed, thesecondary bone anchor 24 is inserted into the bore 40 of the anchor head22 in the manner described above, thereby causing the head 22 to expandand lock against the inner surface of the aperture 274. In particular,the shaft 42 extends into the proximal end of the aperture 274, throughthe anchor head 28, and through the distal end of the aperture 274. Inthis manner, the shaft 42 extends substantially normal to the underlyingbone, while the shaft 26 extends oblique with respect to the underlyingbone. The distal ends of the apertures 274 and auxiliary apertures 274 acan be widened greater than the diameter of the respective shafts asdesired, or can be provided as elongate slots so as to provide forangular flexibility.

Referring now to FIGS. 16A-C generally, while anchor-in-anchor bonefixation systems have been illustrated and described in accordance withvarious fixation procedures, it is recognized that the fixation systemscan also be implemented for spinal fixation, for instance in cervicaland/or lumbar posterior facet screw fixation for management ofinstabilities resulting from, for example, a fracture, a degenerativedisorder, a tumor, or the like.

Referring now to FIGS. 16A-C, the anchor-in-anchor fixation assembly 23of the type described herein can include an auxiliary fixation deviceprovided as an expandable bone plate 352 and a pair of fixation systems,illustrated as fixation systems 20. The bone plate 352 is illustratedwhereby reference numerals corresponding to like elements of bone plate352 are incremented by 100. Thus, the bone plate 352 includes a boneplate body 354 defining a bone-facing surface 353 and an opposingsurface 355, and a pair of apertures 356 extending through the platebody 354.

The bone plate body includes a pair of body segments 363 and 365. Thebody segment 363 includes an inner segment plate 263 a and an outersegment plate 363 b, and an interior groove 367 disposed between theplates 363 a-b that is sized to receive the body segment 365 therein. Atleast one or both inner surfaces 369 that define the groove 367 includeteeth 373 extending out therefrom into the groove 367. Likewise, one orboth of the surfaces of the body segment 365 define teeth 371 extendingoutwardly therefrom and configured to engage the teeth 373. The firstbody segment 363 defines a threaded locking aperture 375 that isthreaded at the outer plate 363 b. Accordingly, the shaft 377 a of athreaded locking pin 377 can be inserted through the aperture 375 in adirection from the inner surface 353 toward the outer surface 355, andengage the threads of the outer plate 363 b so that the head 377 b cancompress the plates 363 a-b against each other, thereby causing theteeth 371 and 373 to engage and prevent relative movement between thebody segments 363 and 365.

During operation, the body segment 365 is inserted into the groove 367,and extended or retracted so that the apertures 356 define a desiredlength therebetween. The second body segment 365 includes a cutout 379that allows the segment 365 to slide without interfering with thelocking pin 377. The locking pin 377 is then rotated within the aperture375 to advance within the aperture 375 and lock the position of the bodysegments 365 and 367. In this regard, it should be appreciated that abone fixation system 20 can be locked within the apertures 356, suchthat the shafts 26 and 42 of the bone anchors 22 and 24 are insertedinto underlying bone in the manner described above. It should beappreciated that once the fixation systems 20 have been attached to theplate 352 and to the underlying bone, the body segment 365 can beretracted within the groove 367 so as to reduce the underlying fracture,or otherwise compress a pair of bones or bone fragments joined to thefixation systems 20 toward each other. Alternatively, the body segment365 can be extended within the groove so as to further space the bonesor bone fragments joined to the fixation systems 20.

The plate 352 can thus be attached to a fractured long bone in themanner described above, or can be used in combination with a corpectomy,whereby a vertebral body is replaced by a vertebral implant. Forinstance, referring now to FIGS. 17A-D, the fixation assembly 23includes the fixation systems 20 and the auxiliary fixation device inthe form of the bone plate 352 along with a second fixation member inthe form of a vertebral implant 300. The implant 300 is illustrated asan annular mesh cage, though it should be appreciated that any vertebralimplant could be incorporated. As illustrated, the implant 300 isdisposed between the fixation systems, and is thus configured to beinserted into an intervertebral space, for instance, after a vertebralbody has been removed.

Once the implant 300 is disposed within the intervertebral space, thefixation systems 20 are affixed to the adjacent vertebral bodies V, forinstance in the cervical spine region, via an anterior approach. Inparticular, both bone anchors 22 and 24 extend into the vertebral body,such that one of the bone anchors extends through the respectivevertebral body V and into one of the pedicles P. In accordance with theillustrated embodiment, the shafts 26 of the first bone anchors 22 areinserted through the corresponding vertebral body V and into a targetone of the pedicles P. Once each shaft 26 has reached its approximatedesired depth in the pedicle P, it is rotated until the central axis C1is aligned with the vertebral body V at the desired angle of entry.Next, the bone fixation plate 352 is attached to the heads 28 in themanner described above with respect to plate 52. For instance, theapertures 356 are fitted over the heads 28 until the heads 28 aredisposed within the apertures 356. Finally, the shafts 42 of the secondbone anchors 24 are inserted into respective heads 28 and into thecorresponding vertebral bodies V. The shafts 42 have a length sufficientso as to not extend into the vertebral foramen VF.

If desired, the plate 352 can be extended or compressed in the mannerdescribed above if it is desired to adjust the spacing between theadjacent vertebral bodies that define the intervertebral space in whichthe implant 300 is disposed. In this regard, it should be appreciatedthat the fixation system 23 allows all bone anchors to be inserted intothe vertebral bodies V via an anterior approach, and that the angularoffset of the bone anchors 22 and 24 define a triangular load bearingplane in the manner described above, and are thus better able towithstand higher forces and prevent subsidence or migration of the boneanchors within vertebral bodies without requiring additional spinalfixation via a posterior approach. The bone anchors 22 can be insertedinto vertically aligned pedicles P as illustrated, or can be insertedinto pedicles P disposed on opposing sides of the vertebral foramen VFdepending on the angular orientation of the central axis C1 of the firstbone anchors 22. Alternatively still, a pair of side-by-side aperturescould be disposed at opposing longitudinal ends of the bone plate 252,such that a pair of fixation assemblies 20 includes a corresponding pairof bone anchors 24 that extend into both pedicles of the respectivevertebra.

Referring now to FIG. 18, a bone plate 452 constructed in accordancewith an another alternative embodiment is illustrated as includingreference numerals corresponding to like structure of bone plate 52incremented by 400. Thus, the bone plate 452 includes a plate body 454.The plate body 454 includes a first segment 454 a and a second segment454 b that is cylindrical and thus rotatable within the first segment454 a. One of the apertures 456 extends through the second segment 454 bat a location off-center with respect to the axis of rotation of thesecond segment 454 b. Otherwise stated, the aperture 456 extendingthrough the second segment 454 b is eccentrically positioned, such thatthe longitudinal position of the bone anchors 22 and 24 mounted in theeccentric aperture 456 is adjustable.

For instance, a first fixation system such as system 20 is affixedwithin the aperture 456 extending through the first segment 454 a in themanner described above, such that the shaft 26 of the first bone anchor22 extends in a direction oblique with respect to the bone plate 452,and the shaft 42 of the second bone anchor 24 extends substantiallynormal with respect to the plate 452, though it should be appreciatedthat both shafts 26 and 42 could alternatively extend in a directionoblique with respect to the bone plate 452. A second fixation systemsuch as system 20 is likewise affixed within the aperture 456 thatextends through the second segment 454 b in the manner described above.Thus, the respective shaft 24 extends in a direction oblique withrespect to the bone plate 452 and the shaft 42 extends substantiallynormal with respect to the bone plate 452, though both shafts 26 and 42could extend oblique with respect to the plate 452. Before or after thefixation assemblies 20 are affixed to the apertures 456, the secondsegment 454 b can be rotated in the direction of Arrow R within thefirst segment 454 a so as to adjust the longitudinal position of therespective shafts 26 and 42 with respect to the shafts 26 and 42 joinedto the aperture 456 extending through the first segment 454 a, therebyincreasing or decreasing the longitudinal distance between the fixationassemblies 20.

While the fixation system 20 is illustrated as coupled to the plate 452,it should be appreciated that, as with all auxiliary fixation devicesdescribed herein, the other fixation system, in this case the secondfixation system 120, could alternatively or additionally be coupled tothe plate 452.

Referring now to FIGS. 19A-C, it should be appreciated that theanchor-in-anchor fixation assembly 23 can include a plurality of bonefixation systems such as bone fixation systems 120 in combination withan auxiliary fixation device provided as a pedicle screw assembly 400.The pedicle screw assembly 400 includes a plurality of pedicle screws402 attached via a fixation rod 404. Each pedicle screw 402 includes aninternal opening 408 configured to receive the fixation rod 404, and alower opening 410 configured to receive the head 128 of the first boneanchor 122. In accordance with the illustrated embodiment, an anchorbody 406 defines both the internal opening 408 and the lower opening410. A collet 412 surrounds the anchor body 406 so as to providecompression against the lower opening 410, and a cap 414 is threadedlyinserted into the upper end of the anchor body 406 so as to providecompression to the internal opening 408, and locking the fixation rod404 in the pedicle screw 402.

During operation, the shaft 126 of the first bone anchor 122 is insertedinto the underlying sacral spine area and into a vertebral body throughthe pedicle, or could alternatively be inserted into a facet, lamina,the spinous process, or alternative vertebral structure as desired, soas to affix the first bone anchor 122 at a location offset with respectto the pedicle. The bone anchor 122 is rotated until the central axis C1is aligned with the pedicle. Next, the shaft 142 of the second boneanchor 124 is inserted through the head 128 and inserted into thepedicle until the head 144 is disposed in the head 128 in the mannerdescribed above. The lower opening 410 is then fitted over the head 128so as to operatively couple the fixation system 120 to the fixation rod404. The angular offset of the shafts 126 and 142 define a triangularload bearing plane in the manner described above, and are thus betterable to withstand higher forces and prevent subsidence or migration ofthe pedicle screw 140 within the vertebral body. As illustrated in FIG.19C, the fixation assembly 23 includes a pair of pedicle screwassemblies 400 mounted onto both opposing pedicles of the fusedvertebral bodies, which can be disposed in any spinal region as desired.

Referring now to FIGS. 20A-C, the anchor-in-anchor fixation assembly 23includes first and second bone anchor-in-anchor fixation systems such assystems 120 coupled to an auxiliary fixation device provided as animplant, such as a shoulder prosthesis 500 configured to mate with ahumeral implant. The prosthetic 500 includes a backing plate 502 that iscoupled to an insert 504. The backing plate can be made from animplant-grade metal such as titanium, while the insert 504 can be madefrom poly-ethylene.

The backing plate 502 defines a backing plate body 506 that presents aconcave insert-engaging surface 508 and an opposing convex surface 510.A pair of spaced apertures 512 extends through the body 506 alongrespective central axes A. A pair of cylindrical extensions 514 projectsdown from the opposing surface 510 at a location aligned with theapertures 512, and each define aperture extensions 512 a. A radialprojection 516 extends inward from the inner surface of each cylindricalextension, and assists in affixing the insert 504 to the backing plate502. A lip 518 projects radially inward from the distal end of eachcylindrical extension 514, and presents a mounting surface for therespective fixation systems 120.

The fixation systems 120 are attached to the backing plate 502 and tounderlying bone by first inserting the shaft 126 of the first boneanchor 122 into underlying scapular bone in the manner described above.Next, the shaft 142 of the second bone anchor 124 is inserted throughthe head 128 and into the underlying scapular bone such that the head144 is disposed inside the head 128 in the manner described above. Next,the backing plate 502 is placed down over the head 128 such that thelips 518 snap over the respective heads 128 and lock the heads 128therein. The shafts 126 can be aligned with the axis A of the aperture512, and the shafts 142 can be angulated with respect to the axis A,though it should be appreciated that both shafts 126 and 142 could beangulated with respect to the axis A.

The insert 504 defines a bearing surface 520 that is concave so as toprovide an artificial or prosthetic glenoid, and an opposing surface522. A pair of locking pegs 524 project down from the opposing surface522 at locations aligned with the apertures 512. The pegs 524 define acircumferential recess 526 that receive the projections radialprojections 516 to attach the insert 504 to the backing plate 504, atwhich point the artificial glenoid 520 provides an articulation surfacefor a humeral implant.

While the concavity of the bearing surface 520 is suitable to provide anartificial or prosthetic glenoid as described above, it should beappreciated that the bearing surface 520 can be provided with anycurvature as desired. For instance, the concavity of the bearing surface520 can be constructed so as to provide a hip cup usable as aball-and-socket joint, usable for instance in hiparthroplasty.

It should be appreciated that a kit can be provided that includes one ormore bone fixation assemblies 23 or components thereof as describedabove. The components of the kit can be configured the same ordifferently. For instance, bone anchors 22 and 24 can be provided havingdifferent lengths and outer diameters of the shafts 42 and 26,differently constructed shafts and threads depending on the needs of thesurgeon and the surgical procedure being performed, and different anglesα defined between the shafts and heads. The kit can further include oneor more auxiliary fixation devices of the type described above.

Although the invention has been described with reference to preferredembodiments or preferred methods, it is understood that the words whichhave been used herein are words of description and illustration, ratherthan words of limitation. For instance, it should be appreciated thatthe structures and features of the various bone fixation assemblies andsystems described herein and their components can be incorporated intoany of the other bone fixation assemblies and systems described hereinand their components, unless otherwise indicated. Furthermore, althoughthe invention has been described herein with reference to particularstructure, methods, and embodiments, the invention is not intended to belimited to the particulars disclosed herein, as the invention extends toall structures, methods and uses that are within the scope of thepresent invention, along with kits having one or more fixation systems,assemblies, or components thereof as described herein. Those skilled inthe relevant art, having the benefit of the teachings of thisspecification, may effect numerous modifications to the invention asdescribed herein, and changes may be made without departing from thescope and spirit of the invention, for instance as recited in theappended claims.

What is claimed:
 1. A method of a securing first and second bone anchorsto an intramedullary nail disposed in a medullary canal of a bone, thefirst bone anchor having a head and a shaft that extends from the headalong a central longitudinal axis, the method comprising the steps of:inserting the first bone anchor into the bone along the centrallongitudinal axis and further into an aperture that extends through theintramedullary nail; inserting the second bone anchor into the bone andinto a bore that extends through the head of the first bone anchor alonga central bore axis of the bore, wherein the central bore axis isangularly offset with respect to the central longitudinal axis.
 2. Themethod as recited in claim 1, wherein the first inserting step furthercomprises inserting the bone anchor into a first portion of the bone,through the aperture of the intramedullary nail, and further into asecond portion of the bone that is opposite the first portion of thebone such that the intramedullary nail is disposed between the first andsecond portions of the bone along the central longitudinal axis.
 3. Themethod as recited in claim 1, further comprising the step of rotatingthe first bone anchor so as to align the central bore axis to a targetlocation of the bone.
 4. The method as recited in claim 1, wherein theshaft of the first bone anchor is threaded, and the method furthercomprises the step of threadably engaging the threaded shaft of thefirst bone anchor into the bone.
 5. The method as recited in claim 1,wherein the second bone anchor includes a second head and a second shaftthat extends from the second head along a second central longitudinalaxis, the second shaft being at least partially threaded, and the secondinserting step further comprises threadedly engaging the second shaftinto the bone.
 6. The method as recited in claim 5, wherein the head ofthe first bone anchor defines an inner surface that is at leastpartially threaded, the inner surface at least partially defining thebore, and the second inserting step further comprises threadablyengaging the second anchor head with the inner surface of the first boneanchor.
 7. The method as recited in claim 1, wherein an outer surface ofthe head of the first bone anchor is threaded, and the first insertingstep comprises threadedly engaging at least a portion of the head of thefirst bone anchor into the bone.
 8. The method as recited in claim 6,wherein the head of the first bone anchor defines a plurality ofrecesses that extend into the inner surface of the head along thecentral bore axis, wherein the second inserting step further comprisesinserting a first portion of the second bone anchor head into one of theplurality of recesses, and inserting a second portion of the second boneanchor into another one of the plurality of recesses.
 9. The method asrecited in claim 1, wherein the bone defines a first bone segment and asecond bone segment that is spaced from the first bone segment, and thefirst inserting step comprises inserting the first bone anchor into thefirst bone segment, the method further comprising the step of driving athird bone anchor into the second bone segment and into an additionalaperture that extends through the intramedullary nail.
 10. The method asrecited in claim 1, wherein the first bone anchor head defines a headproximal end and an opposed head distal end, the head further defining aplurality of retention tabs that extend from the head distal end,wherein the second inserting step further comprises: inserting a firstportion of the second bone anchor head into the bore of the first boneanchor such that the plurality of retention tabs flex radially outward;and inserting a second portion of the second bone anchor further intothe bore of the first bone anchor such that the plurality of retentiontabs can retract and lock the second portion of the second bone anchorinto the head of the first bone anchor.
 11. The method as recited inclaim 1, wherein the first bone anchor and the second bone anchor definean anchor-in-anchor fixation system, the method further comprising thestep of inserting a second anchor-in-anchor system into the bone and asecond aperture of the intramedullary nail, such that respective centralaxes of the respective first bone anchors of the first and secondanchor-in-anchor systems are angularly offset with respect to eachother.
 12. The method as recited in claim 11, further comprising thestep of inserting a third anchor-in-anchor system into the bone and athird aperture of the intramedullary nail, such that respective centralaxes of the respective first bone anchors of the first and thirdanchor-in-anchor systems are aligned.
 13. The method as recited in claim11, further comprising the step of inserting a third bone anchor througha proximal aperture defined by a proximal end of the intramedullarynail.
 14. The method as recited in claim 1, further comprising the stepof inserting the intramedullary nail into a canal of the bone such thatthe intramedullary nail extends across a distal radius fracture.
 15. Themethod as recited in claim 1, wherein the step of inserting the secondbone anchor comprises inserting the second bone anchor into the bonethat surrounds the intramedullary nail without also attaching to theintramedullary nail.
 16. The method as recited in claim 1, wherein thestep of inserting the first bone anchor into the bone and through theaperture further comprises inserting the first bone anchor into anaperture of a bone plate.
 17. The method as recited in claim 16, whereinthe step of inserting the second bone anchor into the bone furthercomprises inserting the second bone anchor into the bore of the firstbone anchor and the aperture of the bone plate.
 18. The method asrecited in claim 1, further comprising a third bone anchor having athird head and a third shaft that extends from the third head along athird central longitudinal axis, the method comprising the steps of:inserting the third bone anchor into the bone along the third centrallongitudinal axis and further into a second aperture that extendsthrough the intramedullary nail such that the third central longitudinalaxis is angularly offset with respect to the central longitudinal axisof the first bone anchor; and inserting a fourth bone anchor into thebone and into a bore that extends through the third head of the thirdbone anchor so that the fourth bone anchor is angularly offset withrespect to third bone anchor.